Our present invention relates to a method of and to an apparatus for the automatic control of continuous casting, i.e. the process whereby a metal, especially molten steel, is delivered by a ladle to a tundish or distributor from which the molten metal is delivered to the shaping part of a continuous casting apparatus. Within this shaping part, the molten metal is at least partially solidified to form a slab.
Continuous casting as defined above can result in slabs of a variety of thicknesses and can use, as the shaping part of the apparatus, a chilled mold which can have broad and narrow sides and can be oscillated, or a travelling mold or shaping member which can, for example, include a roller or even a pair of rollers in a twin roller system, or a belt arrangement on which the metal is deposited. While the invention will be described here primarily in conjunction with a thin slab continuous casting system utilizing an oscillating mold, the principles of this invention are applicable as well to other kinds of continuous casting of steel.
In recent years, especially in thin slab continuous casting, where the casting speed can be say 8 to 10 m/min, automatic control has become increasingly important to insure product quality and reduced down time.
Indeed, the continuous casting process increasingly attempt to reduce the thickness of the solidifying slab and to increase the casting speed. Apart from thin slab casting using oscillating molds as has been mentioned, belt systems with twin roller casting units and even faster operating casting systems have been provided. The solidification times with thinner products are reduced and one can mention, as examples of solidification times.
16 minutes in the case of automatic slab casting with slabs of say 200 mm in thickness.
1 minute in the case of a 50 mm thick so-called thin slab.
0.1 min (0.6 sec) for a 5 mm thick cast strip.
Casting speeds with these systems can be 1 m/min, 10 m/min and 100 m/min respectively.
It has been recognized that it is important for reliable operation of a continuous casting line of the type described and especially for thin slab casting, to acquire data from the various units by on-line measurement and to establish the casting speed which will result in an optimum solidification in the mold simultaneously with reliable and uninterrupted production and good surface quality of the product. Insufficient cooling in the mold can result in breakthrough and excessive cooling in the mold can restrict the production speed and effect the surface quality.
As a consequence, for a 50 mm thick thin slab, the solidification time in the mold should be about 1 minute. Up to now, casting speeds of about 8 m/min have been used to accomplish optimum cooling, although speeds up to 10 m/min are conceivable and have been approached by reliable casting. For casting speeds of the latter magnitude, exact temperature and speed control are required.
Furthermore, the temperature of the steel within the mold must be so selected that the slab surface is liquid at least until just before the product leaves the mold so that the movement through the mold is not impeded, but there should be no presolidification at the level of the molten metal within the mold. The casting powder which is thrown onto the surface of the melt at this level should be sufficiently molten that it can provide effective lubrication between the molten melt and the mold walls and insulation of the continuous casting or strand during its travel through the mold.
It has been recognized that the temperature of the melt within the mold, for an approximately constant temperature of the steel within the ladle, will be dependent on the actual temperature drop within the mold, the residence time of the steel in the distributor or tundish, the insulation of or radiation from the distributor, measured, for example, as the outer skin temperature of the distributor which can be say 100xc2x0 C. at equilibrium over say a 30 minute casting time, the radiation of the heat from the pouring or casting nozzles or tubes (submerged entry nozzles or SEN) and the temperature loss of the steel from the beginning of casting over the approximately 30 minute casting time for the heating up of the tundish from about 1200xc2x0 C. to about 1300xc2x0 C. to 1500xc2x0 C. to achieve a temperature equilibrium between the steel and the tundish.
The residence time of the steel in the tundish itself is determined by the tundish size and capacity and thus the maximum and actual quantities of steel in the tundish and the casting output in terms of casting speed and the width and solidification thickness of the cast strand or slab.
In view of these various influences upon the continuous casting process, it is desirable to be able to effect online or continuous control of that process so that the casting rate for speed is maximized in a reliable way and without the danger of breakthrough or shutdown for repair or premature maintenance.
It is, therefore, the principal object of the present invention to provide an improved method of automatically controlling operation of a continuous casting machine or plant whereby drawbacks of earlier systems are avoided.
It is another object of this invention to provide a continuous casting apparatus with automatic control and hence more efficient operation.
These objects are attained, in accordance with the invention by the acquisition of a simple parameter or group of parameters which allow the temperature conditions in the mold to be predicted or ascertained within a so-called xe2x80x9cincubation timexe2x80x9d and permit an optimum casting speed to be established.
More particularly, the method of the invention automatically controls operation of a continuous casting plant in which the molten steel is fed from a ladle to a tundish distributor having controlled outlets opening into a continuous caster and maintaining a level of cast molten steel and in which the molten steel solidifies into a slab. The method can comprise the steps of:
(a) measuring a temperature Tdist of the molten steel in the distributor;
(b) detecting a temperature loss in the molten steel over a residence time of the molten steel between the distributor and the level;
(c) determining an equivalent liquidus temperature Teliq.+0xc2x0 C. of the steel in the distributor and a plurality of isotherms Teliq.+Zxc2x0 C. thereof; and
(d) controlling a continuous casting speed of the slab so that the temperature Tdist measured in step (a) is maintained within an isotherm window Teliq.+Xxc2x0 C. less than Tdist less than Teliq.+Yxc2x0 C., where Z is greater than X and Y, Xxe2x89xa0Y, and X, Y and Z are integers preferably x, y and z are multiples of 5 and, in a most preferred state, x=5 and y=15.
In apparatus terms, the apparatus for carrying out the method can comprise:
a mold of the continuous caster;
a temperature measuring device in the tundish;
a device for measuring the steel content of the tundish, means for calculating the temperature loss of the steel between the tundish and the mold, means for the online determination of the equivalent liquidus temperature and its isotherms; and
means for controlling the casting speed within the aforementioned isotherm window of the T/VC field, VC being the casting velocity in m/min.
According to a feature of the invention, the method is carried out in a continuous caster with an oscillating casting mold. The coordinates of a melt over the casting time is displayed online in a temperature/velocity (T/VC) scan or system on the computer screen.
It has been found to be advantageous to display the T/VC relationship dynamically as a function of parameters which influence it like the resistance time of the steel in the tundish (dependent upon casting width, casting thickness, true casting speed, true tundish filling height, tundish surface/volume, heating of the tundish from, for example, 1200xc2x0 C. to its equilibrium temperature with the steel of about 1500xc2x0 C., and tundish insulation) in an online display.
According to still another feature of the invention, the temperature measurement in the tundish is carried out continuously or discontinuously or both, presumably together with continuous measurement of the degree of filling of the tundish.
Preferably, the casting velocity is automatically adjusted within the aforementioned window. The preferred window is Teliq+5xc2x0 C. less than Tdist less than Teliq+15xc2x0 C.
The values x and y differ by at least 5 and at most 10 and x, y and z can all be multiples of 5.